1. Field of the Invention
The invention relates to electrical circuit arrangements in general and more particularly to circuits which control the coils of a relay.
2. Prior Art
The use of electrical circuits to generate current for controlling the coils of a relay is well known in the prior art. Relays may be broadly divided into two classes. The first class of relays utilizes current which generates a magnetic field to move a mechanical set of contacts. The contacts may be supported on an open frame arrangement or in a glass enclosed capsule called a reed. The coil surrounds the reed and magnetic force causes the movement of the contacts. Current must flow through the coil continuously in order to maintain the contact.
The second class of relays is the so-called bistable or magnetic latching relay. These relays utilize a holding magnet to hold the contacts in one state or the other, after the contacts have been transferred by passing current through the "set" or "reset" coil. When an electrical current is passed through the "set" coil, the magnet holds the contact in the set state or set position. When an electrical current is passed through the "reset" coil, a force which is greater than that of the holding magnet forces the contacts into the reset state. The set and reset coils are usually wound in opposite directions. Because these relays are bistable, the coils do not require continuous current flow.
Because the invention, to be described below, is primarily concerned with the bistable relays, only prior art which relates to the bistable type relays will be described hereinafter.
U.S. Pat. No. 4,257,081 is an example of the prior art circuit arrangement suitable for controlling a bistable relay. A capacitor is connected in series with the relay. The relay and capacitor are connected across an excitation voltage. A transistor switch is connected in parallel with the relay and the capacitor. When the excitation voltage is activated, current flows through the relay and the capacitor. The current forces the relay into the set state and charges the capacitor. When the excitation voltage is deactivated, the switch discharges the capacitor and resets the relay.
Although the device of the prior art patent works well for its intended purpose, it requires a relatively high current flow and therefore cannot be used in an environment where a high current flow is lacking. Also, there is a possibility that once the relay is set if a malfunction occurs in the circuit, it remains in the set condition. In other words, the relay is not fail-safe. Needless to say, the lack of the fail-safe feature renders the prior art circuit unacceptable for several applications.